Electrical devices comprising conductive polymeric compositions that exhibit a positive temperature coefficient (PTC) effect are well known in electronic industries and have many applications, including their use as constant temperature heaters, thermal sensors, over current regulators and low-power circuit protectors. A typical conductive polymeric PTC composition comprises a matrix of a crystalline or semi-crystalline thermoplastic resin (e.g., polyethylene) or an amorphous thermoset resin (e.g., epoxy resin) containing a dispersion of a conductive filler, such as carbon black, graphite chopped fibers, nickel particles or silver flakes. Some compositions additionally contain non-conductive fillers, such as metal oxides, flame retardants, stabilizers, antioxidants, antiozonants, crosslinking agents and dispersing agents.
At a low temperature (e.g. room temperature), the polymeric PTC composition has a compact structure and resistivity property that provides low resistance to the passage of an electrical current. However, when a PTC device comprising the composition is heated or an over current causes the device to self-heat to a transition temperature, a less ordered polymer structure resulting from a large thermal expansion presents a high resistivity. In electrical PTC devices, for example, this high resistivity limits the load current, leading to circuit shut off. In the context of this invention, T.sub.S is used to denote the "switching" temperature at which the "PTC effect" (a rapid increase in resistivity) takes place. The sharpness of the resistivity change as plotted on a resistance versus temperature curve is denoted as "squareness", i.e., the more vertical the curve at the T.sub.S, the smaller is the temperature range over which the resistivity changes from the low to the maximum values. When the device is cooled to the low temperature value, the resistivity will theoretically return to its previous value. However, in practice, the low-temperature resistivity of the polymeric PTC composition may progressively increase as the number of low-high-low temperature cycles increases, an electrical instability effect known as "ratcheting". Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inorganic fillers or organic additives are usually employed to improve electrical stability.
In the preparation of the conductive PTC polymeric compositions, the processing temperature often exceeds the melting point of the polymer by 20.degree. C. or more, with the result that the polymers may undergo some decomposition or oxidation during the forming process. In addition, some devices exhibit thermal instability at high temperatures and/or high voltages that may result in aging of the polymer. Thus, inorganic fillers and/or antioxidants, etc. may be employed to provide thermal stability.
One of the applications for PTC electrical devices is a self-resettable fuse to protect equipment from damage caused by an over-temperature or over-current surge. Currently available polymeric PTC devices for this type of application are based on conductive materials, such as carbon black filled polyethylene, that have a low T.sub.S, i.e. usually less than 125.degree. C. However, for some applications, e.g. circuit protection of components in the engine compartment or other locations of automobiles, it is necessary that the PTC composition be capable of withstanding ambient temperatures as high as about 120.degree. C. to 130.degree. C., without changing substantially in resistivity. Thus, for these applications, the use of such a carbon black filled polyethylene-based or similar device is inappropriate. Recent interest in polymeric PTC materials, therefore, has focused on selection of a polymer, copolymer or polymer blend that has a higher and sharper melting point, suitable for comprising a high temperature polymeric PTC composition (i.e. a composition having a T.sub.S higher than 125.degree. C.).
For many circuits, it is also necessary that the PTC device have a very low resistance in order to minimize the impact of the device on the total circuit resistance during normal circuit operation. As a result, it is desirable for the PTC composition comprising the device to have a low resistivity, i.e. 10 ohm-cm (.OMEGA.cm) or less, which allows preparation of relatively small, low resistance PTC devices. There is also a demand for protection circuit devices that not only have low resistance but show a high PTC effect (i.e. at least 3 orders of magnitude in resistivity change at T.sub.S) resulting in their ability to withstand high power supply voltages. In comparison with low T.sub.S materials, some high temperature polymeric PTC compositions have been shown to exhibit a PTC effect of up to 10.sup.4 or more. High temperature polymeric PTC compositions also theoretically have more rapid switching times than low T.sub.S compositions, (i.e. the time required to reduce the electrical current to 50 percent of its initial value at the T.sub.S), even at low ambient temperatures. Thus, PTC devices comprising high temperature polymeric PTC materials are desirable because they may be expected to have better performance than low temperature polymeric PTC devices, and also be less dependent on the ambient operating temperature of the application.
High temperature polymeric PTC materials such as homopolymers and copolymers of poly(tetrafluorethylene), poly(hexafluoropropylene) and poly(vinylidene fluoride) (PVDF), or their copolymers and terpolymers with, for example, ethylene or perfluorinated-butyl ethylene, have been investigated as substitutes for polyethylene-based materials to achieve a higher T.sub.S. Some of these compositions exhibited a T.sub.S as high as 160-300.degree. C. and a resistivity change at T.sub.S of up to four orders of magnitude (10.sup.4) or more. However, thermal instability and the potential for release of significant amounts of toxic and corrosive hydrogen fluoride if overheating occurs, has restricted these materials from practical consideration for high temperature applications.
A variety of other polymers have been tested to explore PTC characteristics. These polymers include polypropylene, polyvinylchloride, polybutylene, polystyrene, polyamides (such as nylon 6, nylon 8, nylon 6,6, nylon 6,10 and nylon 11), polyacetal, polycarbonate and thermoplastic polyesters, such as poly(butylene terephthalate) and poly(ethylene terephthalate). Under the conditions reported, none of these polymers exhibited a useful high temperature PTC effect with a low resistivity state of 10 .OMEGA.cm or less. However, it has been reported that the PTC characteristics of certain crystalline polymers, such as polyethylene, polypropylene, nylon-11, and the like, may be improved if they are filled with electrically conducting inorganic short fibers coated with a metal.
More recently, a novel high temperature polymeric PTC composition comprising a polymer matrix of an amorphous thermoplastic resin (crystallinity less than 15%) and a thermosetting resin (e.g. epoxy) has been described. Because the selected thermoplastic resin and thermoset resin were mutually soluble, the processing temperature was substantially low and depended on the curing temperature of the thermoset resin. The use of a thermoset resin apparently assured sufficient crosslinking and no further crosslinking was employed. However, electrical instability (ratcheting) was still a problem with these compositions.
For the foregoing reasons, there is a need for the development of alternative polymeric PTC compositions, and PTC devices comprising them, that exhibit a high PTC effect at a high T.sub.S, have a low initial resistivity, are capable of withstanding high voltages, and exhibit substantial electrical and thermal stability.
In our copending U.S. patent application Ser. No. 08/729,822, filed Oct. 8, 1996, we disclose a high temperature PTC composition and device comprising nylon-12 and a particulate conductive filler such as carbon black, graphite, metal particles and the like. The composition demonstrates PTC behavior at a T.sub.S greater than 125.degree. C., typically between 140.degree. and 200.degree. C., more typically between 150.degree. C. and 190.degree. C., a high PTC effect (a maximum resistivity that is at least 10.sup.3 higher than the resistivity at 25.degree. C.), and a low initial resistivity at 25.degree. C. of 100 .OMEGA.cm or less (preferably 10 .OMEGA.cm or less). The entire disclosure of the copending application is hereby incorporated by reference.